BQ77915: Balancing in stacked topology

Also can you get me some more information about the BQ77915 internal MOSFETs. The data is scattered and seems to be inconsistent.

For example the BQ77915 says it is 12ohms typical at 4V.

The BQ29312/330 seems be be 330ohm at 2.24V, 260ohm at 2.6V and 220ohm at 2.76V from App note SLUA420A.

This is consistent with the data sheet for BQ77PL900 and the BQ29312 and BQ29330.

Do all the balancing circuits from TI have the same internal balancing MOSFETs?

I would guess that for best results alternate cells should balance, particularly when external MOSFETs are used.

Thanks.

Hi Paul,

If you have multiple stacked devices, the devices will balance their cells independently. If you have a 10S configuration, we would recommend connecting 5 cells to the bottom device and 5 cells to the top device. 

Not all of our devices use the same internal components. The internal MOSFETS in the BQ77915 are 12 Ohm typical.

Thank you for your reply.

In the end I worked it out for myself.

There is a gap in the TI knowledge base for the BQ77915, because it is using a fundamentally different method to drive its internal MOSFETs. Some older balancing chips drive each internal MOSFET from the high side of the cell directly. But the BQ77915 doesn't. It can't because it has such a low Rds on. It has a booster, or storage cap on each MOSFET gate drive. And that's why it needs to pulse the balancing - to keep toping up the gate drive.

Somewhere in the documentation you should say this, because otherwise someone such as myself sees the old literature and torments themselves getting information about the MOSFET to figure out if they have enough gate drive for their external MOSFET.

I actually have an 11 cell pack. I want to drive using 4 on bottom, 4 in middle and 3 at top.

Paul.